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Extractor (Matcher)

The main component of rare is the extractor (or matcher). There are three fundamental concepts around the parser:

  • Each line of an input (separated by \n) is matched to a regex
  • A regex is used to parse a line into a match (and optionally, groups)
  • An expression (see: expression) is used to format an output from a regex group
  • Optionally, one or more ignore filter can be applied to silent matches that satisfy a truthy-comparison

Decomposing a Filter

The most primitive way use rare is to filter lines in an input. We'll be using an example nginx log for our example.

Nginx log line looks like this: - - [14/Apr/2016:18:13:29 +0200] "GET / HTTP/1.1" 206 101 "-" "curl/7.43.0"

So, to parse this we may want to match the method and path with a regex:

rare filter -m '"(\w{3,4}) ([A-Za-z0-9/.@_-]+)' access.log

This will extract the method and url and output the entire line to the screen (if matched).

If you want it to only output the matched portion, you can add -e "{0}"

Lastly, lets say we want to ignore all paths that equal "/", we could do that by adding an ignore pattern: -i {eq {1} /}


Histograms are like filters, but rather than outputting every match, it will create an aggregated count based on the extracted expression.

So, with the same example as above, if we extract the method and url, we will get something that will count based on keys.

rare histogram -m '"(\w{3,4}) ([A-Za-z0-9/.@_-]+)' -e '{1} {2}' -b access.log